Marking method and marked molding

ABSTRACT

A method for readily applying a marking to an object made of PTFE, for example, a seal ring provided with a cut portion on a part of a circumference, or an intermediate product for the seal ring, comprising the steps of preparing an object to be marked that is made of PTFE, irradiating a laser beam onto the object to be marked to alter the state of only an irradiated surface of the PTFE, and forming a marking with a white-based color on the irradiated surface of the PTFE so that the irradiated surface exhibits a color tone different from that of a non-irradiated surface of the PTFE.

BACKGROUND OF THE INVENTION

This invention relates to a marking method and a product marked by themarking method, and more particularly relates to a method for applying amarking to an object to be marked by irradiating a laser beam onto theobject and a product obtained by the method. Also, the present inventionis utilized when identification is required in a technical field forusing or producing PTFE (polytetrafluoroethylene).

Heretofore, it has been required for a marking to be applied to asurface of a resin-molded product in order to identify the product bymeans of a type or a lot number of an article.

There are known conventional marking methods for a resin material, suchas a method for forming a marking by irradiating a laser beam onto athermoplastic resin material to make a foamed bulge (Japanese PatentLaid Open No. 62-13287); a method for forming a marking by irradiating alaser beam onto a surface of a transparent electrode substrate to meltthe substrate surface (Japanese Patent Laid Open No. 9-175025); a methodfor forming a marking by coating a surface of an object with resin paintto make a resin film layer, and irradiating a laser beam onto the resinfilm layer to discolor it by means of thermal change without peeling theresin film layer (Japanese Patent Laid Open No. 4-42122); a method forforming a marking by irradiating a laser beam onto a resin compositioncontaining an additive to discolor and foam the additive (JapanesePatent Laid Open No. 2001-113830); a method for forming a marking byirradiating a laser beam onto a plastic material containing carbon blackor graphite as a filler to discolor the filler (Japanese PatentPublication No. 61-11771); a method for forming a marking by irradiatinga laser beam onto a thermoplastic resin material containing borate tofoam or resolve it (Japanese Patent Laid Open No. 8-142510); a methodfor forming a marking by irradiating a laser beam onto a thermoplasticresin material containing carbon black, iron oxide, ultravioletabsorbent, or the like as a filler (Japanese Patent Laid Open No.2001-294758). However, since these methods are to form the marking bypeeling a part of a coating film or by mixing a foaming agent or adiscoloring agent into a resin material, the methods have a problem ofcausing foreign substances, a problem of increasing a man-hour, aproblem of requiring a special compounding agent, or a problem oflimiting a range of composition of a material which can be marked. Inaddition, these methods neither disclose nor suggest application of amarking on PTFE.

On the other hand, some conventional methods for applying a marking toPTFE employ a technique of paint coating or stamping. However, themarking method employing the technique of using paint has followingproblems.

-   (1) Since PTFE has repellency against water and oil on its surface    due to its material characteristics, it has a poor wet property for    paint and thus paint cannot be applied uniformly to PTFE. This    results in an uneven or unclear marking.-   (2) Since PTFE has a poor adhesive property on other materials,    marking paint will be easily stripped from PTFE. The product thus    cannot be identified and the stripped paint will cause foreign    substances. Consequently, such marked product will not satisfy a    requirement for an essential property of an article.-   (3) In the case of coating PTFE with paint, since a paint layer    increases partially a thickness of the PTFE, such marked product    will not satisfy a requirement for an essential property of an    article.-   (4) It is required for a long period in time to dry a dispersion    medium such as water or organic solvent used in paint.-   (5) In the case where a marking surface is not flat (for example, a    curved surface, or a polygonal surface), it is impossible to apply    printing with paint on that surface.-   (6) Although it is necessary to add a special additive for the    purpose of marking, the additive is not necessary for a product in    view of an essential property of the product. Accordingly, it is    necessary to suitably select an additive to be used in accordance    with an application and a function of a product or a finished    article. This will require a lot of man-hours for steps of selection    and addition. Moreover, as stated above, since the additive is not    necessary for a product in view of an essential property of the    product, the additive may cause contaminants.

Accordingly, an aim of the present invention is to provide a method forreadily applying a marking to an object to be marked which is made ofPTFE and to provide a product obtained by the method.

Still other aims and advantages of the invention will in part be obviousand will in part be apparent from the specification.

SUMMARY OF THE INVENTION

To achieve the above aims, the present invention according to claim 1 isdirected to a method for applying a marking to an object to be marked byirradiating a laser beam onto the object. The method comprises the stepsof: preparing an object to be marked which is made of a material ofPTFE; irradiating a laser beam onto the object to be marked to alter thestate of only an irradiated surface of the PTFE; and forming a markingwith a white-based color on the irradiated surface of the PTFE so thatthe irradiated surface exhibits a color tone different from that of anon-irradiated surface of the PTFE.

The invention of claim 2 is directed to a product marked by a method forapplying a marking to an object to be marked by irradiating a laser beamonto the object, comprising the steps of: preparing an object to bemarked which is made of a material of PTFE; irradiating a laser beamonto the object to be marked to alter the state of only an irradiatedsurface of the PTFE; and forming a marking with a white-based color onthe irradiated surface of the PTFE so that the irradiated surfaceexhibits a color tone different from that of a non-irradiated surface ofthe PTFE.

The features of the present invention are as follows.

-   (1) It is possible in the present invention to apply a marking to    PTFE that contains no filler. When the marking method of the present    invention is applied to the PTFE, an irradiated portion of the PTFE    is altered (to cause fluffing) while a non-irradiated portion of the    PTFE is not altered (not to cause fluffing). Although both portions    exhibit the same white colors, which are slightly short of    identification, the irradiated portion can be distinguished from the    non-irradiated portion on account of different degrees of reflection    in light between them.-   (2) It is possible in the present invention to apply a marking to    PTFE that contains any filler.    -   a) In the case where a filler has a color other than a        white-based color:

If the marking method of the present invention is applied to PTFE thatcontains a filler which has a color other than a white-based color, amarking is excellent at identification, since the irradiated portion ofthe PTFE is altered (to be fluffed) while exhibiting its own white colorand exhibits a distinct contrast.

-   -   b) In the case where a filler has a white-based color:

The marking method of the present invention can be applied to a fillerthat has a white-based color. If the marking method of the presentinvention is applied to PTFE that contains a filler which has awhite-based color, a difference between the irradiated portion and thenon-irradiated portion is present on account of different degrees ofreflection in light, although the irradiated portion is slightly poor inidentification.

It is preferable to select a filler from materials such as glass fiber,carbon fiber, glass bead, carbon powder, potassium titanate fiber,alumina powder, bronze powder, silica powder, boron nitride powder,molybdenum disulfide powder, silicon nitride powder, aromatic polyesterpowder, polyetheretherketone powder, polyimide powder. It should benoted that the filler is not limited to the above materials.

In the case where an object to be marked is made of PTFE, when theobject is irradiated by a laser beam, PTFE is fluffed and a surface ofthe object is covered with PTFE to exhibit its own white-based color.Thus the function of a marking is achieved. It is preferable to set anamount of PTFE that covers the surface of the object in accordance witha hue of a base material (a hue of an object to be marked) to identifythe marking.

The marking method of the present invention does not employ aconventional technique of peeling a paint material, peeling a surfacelayer, discoloring or color-developing a composition such as a specialfiller, or foaming a blowing agent but employs marking with awhite-based color by means of fluffing. PTFE can be selected inaccordance with fluffing regardless of a kind and an amount of a filler.Accordingly, it is possible to select a mixture of PTFE and other resinmaterial or elastomer within a range of composition that the PTFE isfluffed. Also, the marking method of the present invention isadvantageous, since no foreign substances are caused during irradiationof a laser beam.

Principles in conventional marking methods are explained by thefollowing three patterns in FIGS. 3A, 3B and 3C.

-   (1) Peeling of a printed surface (a contrast between an upper layer    52 and a lower layer 53 is present by peeling an ink or a paint 51    on the upper surface 52 (see FIG. 3A).-   (2) Peeling of a surface layer (a surface layer 54 is peeled by a    laser beam (see FIG. 3B).-   (3) Color development due to chemical change (a surface 55 is    brought into color development by causing a chemical change by means    of irradiation of a laser beam (see FIG. 3C).

It should be noted that the present invention is not based on any of theabove principles. The present application includes the followingembodiments.

-   (1) A marking with a white-based color on PTFE by utilizing a laser    apparatus.-   (2) A product marked by irradiating a laser beam onto PTFE.-   (3) A method for applying a marking to PTFE by irradiating a laser    beam.-   (4) A marking with a white-based color on PTFE that contains a    filler by utilizing a laser apparatus.-   (5) A product marked by irradiating a laser beam onto PTFE that    contains a filler.-   (6) A method for applying a marking to PTFE that contains a filler    by irradiating a laser beam.-   (7) A method for applying a marking to an object to be marked by    irradiating a laser beam onto the object, comprising the steps of:    preparing an object to be marked which is made of a material of    PTFE; irradiating a laser beam onto the object to be marked to alter    the state of only an irradiated surface of the PTFE; and forming a    marking with a white-based color on the irradiated surface of the    PTFE so that the irradiated surface exhibits a color tone different    from that of a non-irradiated surface of the PTFE.-   (8) A product marked by a method for applying a marking to an object    to be marked by irradiating a laser beam onto the object, comprising    the steps of: preparing an object to be marked which is made of a    material of PTFE; irradiating a laser beam onto the object to be    marked to alter the state of only an irradiated surface of the PTFE;    and forming a marking with a white-based color on the irradiated    surface of the PTFE so that the irradiated surface exhibits a color    tone different from that of a non-irradiated surface of the PTFE.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention believed to be novel and theelements characteristic of the present invention are set forth withparticularity in the appended claims. The figures are for illustrationpurposes only and are not drawn to scale. The invention itself, however,both as to organization and method of operation, may best be understoodby reference to the detailed description which follows taken inconjunction with the accompanying drawings in which:

FIG. 1 is an enlarged photographical view of a surface of a PTFEcomposition, which is marked by a marking method according to anembodiment of the present invention;

FIG. 2 is a perspective view of a seal ring, which is marked by amarking method according to an embodiment of the present invention; and

FIGS. 3A, 3B, and 3C are explanatory views, each illustrating aprinciple of a conventional laser marking method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, embodiments of the present invention will benow explained below.

A laser beam source to be used in a marking method of the presentinvention may be, for example, a gas laser such as a CO₂ laser, asemiconductor laser, a solid body laser such as a YAG laser, or thelike. An amount of a thermal irradiation for marking depends on a degreeof identification, although there are some differences among kinds oflaser. The present invention will be described below in more detail inconnection with a condition of irradiation using a CO₂ laser.

A PTFE product, which is used as an object to be marked, is a sheet (20mm×20 mm×2 mm) obtained by pressing and baking a mixture in which 17parts by weight of carbon fiber having a length of 100 μm are dispersedin 100 parts by weight of PTFE. The sheet has a black-based color.

FIG. 1 shows an electron micrograph of a marking applied under acondition of 3.6 W in power and 300 mm/s in marking speed. In FIG. 1, anupper layer shows a portion irradiated by a laser beam while a lowerlayer shows a non-irradiated portion. The words “to alter PTFE byirradiating a laser beam (to cause fluffing)” described herein means thestate of the upper layer shown in FIG. 1.

Next, conditions and results in examples of a marking according to theembodiment are shown in Table 1.

As shown in Table 1, when a marking speed is fixed at 150 (mm/s), amarking exhibits a distinct white-based color at 1.2 W or more of power.

Further, when a marking speed is fixed at 300 (mm/s), a marking exhibitsa distinguishable white-based color with fluffing at about 2 W of powerand exhibits a distinct white-based color at 3.6 W or more of power.

Still further, when a marking speed is fixed at 400 (mm/s), a markingexhibits a distinguishable white-based color with fluffing at about 2 Wof power and exhibits a distinct white-based color at 4.8 W or more ofpower.

However, since an irradiated portion is subject to change in surfaceroughness and thickness in comparison with a non-irradiated portion, asurface state of the irradiated portion may exert an influence upon afunction of an article. For example, when a CO₂ laser, in which adiameter of a laser beam is 14 mm, is used to apply a marking to a sealring, the seal ring is subject to the following effects.

-   (1) Deformation, for example, deterioration of circularity, change    of thickness;-   (2) Concave and undulation of surface-   (3) Deterioration of physical properties in a surface layer    (mechanical strength, elongation, energy of rupture, abrasion    resistance and the like)-   (4) Abnormality in frictional phenomenon upon sliding.

The above effects may cause a possibility of deterioration of aleak-proof performance. Accordingly, even if a marking is in adistinguishable state, it may be not suitable to an article.

In view of the above evaluation on Table 1, it will be preferable toapply a marking to an object to be marked by increasing a power of laserand a marking speed in order to obtain an excellent marking withoutaffecting a primary function of an article.

Accordingly, it will be preferable from Table 1 that a marking conditionin the present embodiment is 1.2 W of power at 150 mm/s of a markingspeed, 3.6 to 6 W of power at 300 mm/s of a marking speed, and 4.8 to8.4 W of power at 400 mm/s of a marking speed.

It is possible to control a marking state (contrast) by regulating apower of an irradiating laser beam and a marking speed. That is, if thepower is lowered and the marking speed is increased, the marking state(contrast) is attenuated while if the power is raised and the markingspeed is decreased, the marking state (contrast) is deepened.

In the case of applying a laser beam to an object to be marked which ismade of PTFE, a surface temperature on the object upon irradiation ofthe laser beam will be estimated by the following equation:T _(p)=2βF/({square root}(πλcp))×{square root}(t)

-   -   T_(p): surface temperature (° C.)    -   β: absorption factor of laser    -   F: power density (W/cm²)    -   λ: coefficient of thermal conductivity (cal/cm ° C. s)    -   c: specific heat (cal/° C. g)    -   p: density (g/cm³)    -   t: irradiating period in time (s)

A product, which is made of PTFE, may be, for example, a seal ring, abackup ring, a thrust washer, a slipper seal, a wear ring, a bellows, ora sheet. It should be noted that the above products are not limited. Amarking method of the present invention can be applied to anintermediate product or an end product of each of the above products.For example, an intermediate product for the seal ring is a productbefore being cut while an end product for the seal ring is a productafter being cut.

In a seal ring 1 that is provided with a cut portion 2, as shown in FIG.2, it is possible to identify the cut portion 2 by applying a marking 3with a white-based color to the cut portion 2 before hand. Such marking3 can be used to position the cut portion 2. The cut portion 2 isutilized as a fitting part when assembling. The cut portion 2 is formedinto a straight cut or a stepped cut as well as a bias cut shown in FIG.2.

In the case where the seal ring 1 is made of PTFE including a filler ina dark color, when an outer peripheral position on the cut portion 2 isirradiated by a laser beam in accordance with the marking method of thepresent invention, a PTFE portion is fluffed, as shown in FIG. 1,thereby applying to the cut portion 2 the marking 3 that has a primarywhite-based color of PTFE.

Accordingly, it is possible in the seal ring to easily identify the cutportion 2 that serves as a fitting part, to identify similar seal rings,or to identify orientation of the seal ring. Also, in the case ofapplying a marking with a white-based color to an intermediate productfor the seal ring 1, a marking work can be done simultaneously to aplurality of them.

Although the marking 3 is depicted in a simple band-like pattern in FIG.2, various kinds of characters, signs, figures or the like such as aname of a company, an article code, or the like can be marked.

The irradiated and fluffed portion with a white base color is returnedto the original color by means of friction, because a fluffed state ofthe PTFE is returned to the original dense state color. Accordingly,when a marking is applied to a sliding portion, the marking serves as asignal on a product but the marking is abraded away by the abovereaction after the product is transformed into a finished article andthe finished article is assembled in a sliding portion. In other words,the marking will be good enough to remain effective (not to be abradedaway) from a producing step to a assembling step. Even if the marking isworn after assembling, this will not affect a primary function of thefinished article. If a marking is applied to a non-sliding portion of aproduct, various kinds of characters, signs, figures or the like such asa name of a company, an article code, or the like can be applied to theportion. If the marking is applied to a non-sliding portion of aproduct, since the marking is not abraded away, it is possible toidentify a finished article and not to affect a function of the article.

The present invention has the following advantages.

In a marking method according to claim 1 and a product according toclaim 2, it is possible to readily apply a marking with a white-basedcolor to an object to be marked, which is made of PTFE. Accordingly,when it is necessary to identify a product made of PTFE, it is possibleto easily perform an identifying work.

According to the marking method of the present invention, it is possibleto directly apply a marking to a product without working a surface ofPTFE that is an object to be marked. Consequently, it is possible toform a uniform and distinct marking.

According to the marking method of the present invention, since themarked portion is not peeled and foreign substances due to peeling arenot caused, the marking does not affect a primary function of the markedproduct.

Accordingly, a marking of the present invention can be applied to asliding portion of a product.

Since it is not necessary to dry a marked portion in the presentinvention, a period in time required for marking can be shortened.

In the present invention, when an object to be marked is irradiated by alaser beam, since an irradiated portion of PTFE is altered (fluffed) andthe portion of the object exhibits a white-based color that is a primarycolor of PTFE, thereby the function of a marking being achieved, it ispossible to apply a marking to any surface on the object that is notflat.

In the marking method of the present invention, since it is not requiredto add a special additive for the purpose of marking, which isunnecessary for a primary function of a product or a finished article,it is possible to omit a work of selecting an additive for every objectsto be marked or adding an additive to the object. Also, it is possibleto avoid generation of contaminants from a product or a finishedarticle.

The present invention can further obtain the following effects.

-   (1) It is possible to easily control a contrast with a white-based    color by regulating a power and an irradiating speed of a laser    apparatus.-   (2) It is possible to apply a marking with small characters to an    object to be marked, since it is possible to readily carry out a    control of a marking size.-   (3) In comparison with a conventional technique, the present    invention is not subject to limitation of peeling of paint, peeling    of a surface layer, discolor or color development of a composition    such as a special filler or the like, and mixing of a special    forming agent, PTFE is not subject to limitation of composition.

While the present invention has been particularly described, inconjunction with a specific preferred embodiment, it is evident thatmany alternatives, modifications, and variations will be apparent tothose skilled in the art in light of the foregoing description. It istherefore contemplated that the appended claims will embrace any suchalternatives, modifications, and variations as falling within the truescope and spirit of the present invention.

1. A method for applying a marking to an object to be marked byirradiating a laser beam onto said object, comprising the steps of:preparing an object to be marked that is made of a material of PTFE;irradiating a laser beam onto said object to be marked to alter thestate of only an irradiated surface of the PTFE; and forming a markingwith a white-based color on the irradiated surface of the PTFE so thatthe irradiated surface exhibits a color tone different from that of anon-irradiated surface of the PTFE.
 2. A product marked by a method forapplying a marking to an object to be marked by irradiating a laser beamonto said object, comprising the steps of: preparing an object to bemarked that is made of a material of PTFE; irradiating a laser beam ontosaid object to be marked to alter the state of only an irradiatedsurface of the PTFE; and forming a marking with a white-based color onthe irradiated surface of the PTFE so that the irradiated surfaceexhibits a color tone different from that of a non-irradiated surface ofthe PTFE.